Cu(In, Ga)Se-2 (CIGS) solar cells were fabricated by selenizing Cu-In-Ga precursors obtained by sputtering a Cu-In-Ga ternary alloy target. A preheating process was introduced during the selenization process to optimize the interface between the absorption layer and the buffer layer to improve the device performance. The results showed that the quality of the CIGS/CdS interface changed significantly with different preheating temperatures. A smoother interface indicated better p-n junction performance because of a more uniform distribution and better coverage of the buffer layer and intrinsic ZnO, as well as less interface recombination. Furthermore, there is less residue of Cu-Se compounds as the preheating temperature increases. Finally, the highest device efficiency of 14.35% was achieved at a preheating temperature of 300 degrees C.